Olefin polymers, especially polypropylene, are commonly spun into fibers, film or sheet by extruding molten polymer through die orifices such as a spinnerette, film or sheet dies, quenching the molten filament, film or sheet, orienting the filament, film or sheet, and heat setting the oriented filament, film or sheet prior to winding upon a bobbin. Woven and non-woven fabrics are commonly made from such filament, film or sheet.
A problem associated with polyolefins produced using Ziegler-Natta type catalysts is the generation of visible "smoke" during melt extrusion of polyolefin during fiber spinning and film extrusion operations. The "smoke" evolves at the die, and is believed to comprise volatile organic compounds in the 20-50 carbon atom range. These evolved volatile organic compounds subsequently condense and coat equipment with resultant problems of non-uniformity of nonwoven fabrics formed from such fibers or of films due to die deposits caused by the volatile organic compounds. Efforts to reduce or eliminate smoke have been partially successful at best. Minimizing both melt temperature and extruder residence time below certin levels can create processing problems. Devolatizing the polymer feedstock by heating prior to extrusion can reduce smoke generation by about 20% In contrast, the present invention can reduce smoke by up to 86% percent or more.
The incorporation of various additives into and/or onto olefin polymer materials to improve thermal stability, UV resistance, and processability is known. For example, the inclusion of an acid neutralizing agent in olefin polymer compositions is necessary due to small amounts of catalyst residues contained within the olefin polymer. These catalyst residues can cause corrosion of processing equipment such as mold surfaces and die lips. The addition of an appropriate acid neutralizing agent can eliminate or at least reduce the potential for corrosion due to such residues.
The selection of acid neutralizing agent is important because it can affect the overall acidity/basicity of an olefin polymer composition and can influence the reactions of many of the organic additives in the polymer composition. In addition, the polyolefin's release properties can be affected by the acid neutralizing agent.
In practice, metallic stearates such as sodium, calcium and zinc are commonly added to olefin polymer materials as an acid neutralizing agent, with calcium stearate being the most common. Calcium stearate is predominately used because it also functions as an external lubricant and processing aid in addition to acting as an acid neutralizing agent. Generally, it is necessary to add the calcium stearate in an amount of at least 500 ppm to ensure that it can function as an effective acid neutralizing agent.
Phosphite compounds, including 2,2',2"-nitrilo[triethyl-tris(3,3',5,5'-tetra-tert. butyl-1,1-biphenyl-2,2'-diyl)phosphite] are typically added to polyolefin compositions to stabilize them against thermal and oxidative degradation. Various forms of 2,2',2"-nitrilo[triethyl-tris(3,3',5,5'-tetra-tert. butyl-1,1-biphenyl-2,2'-diyl)phosphite] are known. For example, U.S. Pat. No. 5,326,802 discloses a beta crystalline modification of 2,2',2"-nitrilo[triethyl-tris(3,3',5,5'-tetra-tert. butyl-1,1-biphenyl-2,2'-diyl)phosphite]. Example 6 discloses the stabilization of polypropylene which also contains 750 ppm calcium stearate. U.S. Pat. Nos. 5,331,031 and 5,405,893 disclose a gamma crystalline modification of 2,2',2"-nitrilo[triethyl-tris(3,3',5,5'-tetra-tert. butyl-1,1-biphenyl-2,2'-diyl)phosphite]. Example 4 illustrates the stabilization of polypropylene which also contains 750 ppm calcium stearate. An amorphous solid modification of 2,2',2"-nitrilo[triethyl-tris(3,3',5,5'-tetra-tert. butyl-1,1-biphenyl-2,2'-diyl)phosphite] is disclosed by U.S. Pat. No. 5,276,076. Example 3 illustrates the stabilization of polypropylene which also contains 750 ppm calcium stearate.
The use of N,N-dialkylhydroxylamines to stabilize polyolefins is also known. Thus, U.S. Pat. No. 4,668,721 discloses that hydroxylamine derivatives may be used to stabilize polyolefin compositions against degradation due to extrusion, exposure to the combustion products of natural gas, gamma radiation or upon storage. The hydroxylamine derivative may have a chemical structure which conforms to one of fourteen structural formulae. In Example 21, a combination of hydroxylamine and calcium stearate (1000 ppm) is said to be far superior to hydroxylamine alone with respect to the yellowing resistance of polypropylene processed at 260.degree. C.
U.S. Pat. No. 4,876,300 discloses that long chain N,N-dialkylhydroxylamines can be used as process stabilizers for polyolefin compositions in order to minimize discoloration and melt flow rate increase due to extrusion. Examples 16-18 and 20 illustrate the process stabilization of polypropylene which contains 1000 ppm calcium stearate, while Example 19 illustrates the process stabilization of polypropylene containing 1000 ppm zinc stearate.
International Patent Publication No. WO 94/24344 discloses a polypropylene stabilized with an effective amount of a selected hindered amine, a selected N,N-dialkylhydroxylamine, and a phosphite which may be 2,2',2"-nitrilo[triethyl-tris(3,3',5,5'-tetra-tert. butyl-1,1-biphenyl-2,2'-diyl)]phosphite. The polypropylene composition is free or essentially free of any traditionally used phenolic antioxidant, and is said to possess enhanced light stability, enhanced long term heat stability and especially enhanced gas fade stability. All of the formulations disclosed in the examples contain 750 ppm calcium stearate.
An object of the invention is to provide a stabilized olefin polymer composition which exhibits good processing characteristics and yet which generates a minimum amount of smoke during extrusion into a fiber or film.
Yet another object of the invention is to provide a method for reducing volatile organic compounds generated during polyolefin fiber and film production by up to 86 percent.